The present invention relates to a master cylinder which generates a fluid pressure for operating a brake apparatus.
As a conventional master cylinder, there can be mentioned a master cylinder disclosed in Unexamined Japanese Patent Application Public Disclosure No. 2-136363. This master cylinder comprises a cylinder, a piston slidably provided in the cylinder, an annular cup seal held on an inner circumferential surface of the cylinder, and a relief port formed in the piston. The piston and the cylinder define a pressure chamber for generating a fluid pressure on a side to which the piston advances. The cylinder and an outer circumferential surface of the piston define a fluid supply chamber which communicates with a reservoir. The cup seal is adapted to be slid against by the straight outer circumferential surface of the piston to separate the fluid supply chamber from the pressure chamber. The relief port has two openings. One of the two openings is open on the outer circumferential surface of the piston, while the other opening is always open to the pressure chamber. Thus, the relief port is capable of providing communication between the pressure chamber and the fluid supply chamber.
In this master cylinder, the piston needs to move (or advance) a large distance (invalid stroke) from its initial position to the position where the relief port is closed by the cup seal. Due to limitation on the magnitude of the invalid stroke, the relief port of the piston at the initial position cannot be displaced sufficiently rearwardly from a rear end portion of the cup seal, making it difficult to communicate the relief port with the fluid supply chamber to a satisfactorily large degree.